Grinding wheel and method for manufacturing the same

ABSTRACT

A grinding wheel includes a plurality of super abrasive grains arranged so as not to contact one another; a vitrified bond formed like a bridge that bridges the plurality of super abrasive grains one another, and bonding the plurality of super abrasive grains one another; a plurality of fine grains arranged in the vitrified bond while forming a group and interposed between the plurality of super abrasive grains so that the plurality of super abrasive grains are arranged so as not to contact one another; and pores formed around the vitrified bond.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2015-090292 filed on Apr. 27, 2015 including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a grinding wheel and a method for manufacturing the same.

2. Description of Related Art

To prevent thermal damage such as grinding burn and grinding cracks and to achieve high machining accuracy, a grinding wheel with small grinding resistance is used for efficient grinding. In this case, a low-concentration grinding wheel having a reduced abrasive-grain content (abrasive-grain concentration) per unit volume is effectively used to reduce the grinding resistance. Typically, the low-concentration grinding wheel is formed by mixing abrasive grains, an aggregate, and a bond, and then by molding and sintering the mixture. The aggregate and the bond are arranged between the abrasive grains. The aggregate arranged between the abrasive grains is often made of, for example, white alumina (WA). In the case where the WA is used, however, as grinding of a workpiece performed by the grinding wheel proceeds, abrasion of the aggregate also proceeds by chips from the workpiece, flattening a surface of the aggregate on the workpiece side and forming a smoothed portion. The smoothed portion increases the grinding resistance. The increase of the grinding resistance may cause significant crush or falling-off of the aggregate. Thus, the bond that is a bond bridge, for example, may be destroyed so that CBN abrasive grains fall off to shorten the tool life.

To solve the above problem, a technique is described in Japanese Patent No. 5398132 (JP 5398132). In the technique of JP 5398132, the aggregate is made of porous ceramic having a large number of small holes. Thus, even when grinding of a workpiece performed by the grinding wheel proceeds along with the abrasion of the aggregate, and then the destruction of the aggregate starts, the destruction is suppressed within a portion up to a small hole formed in the aggregate. Accordingly, small destruction up to a small hole is repeatedly caused in the porous aggregate, which prevents significant and rapid destruction of the porous aggregate. However, controlling the arrangement of the holes in the porous ceramic is very difficult. Thus, once the destruction starts, it is not ensured that the destruction is reliably suppressed into the small destruction.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a grinding wheel that includes a member for adjusting a distance between the abrasive grains and that has a long life, and is to provide a method for manufacturing the grinding wheel.

According to a first aspect of the present invention, a grinding wheel includes:

-   -   a plurality of abrasive grains arranged so as not to contact one         another;     -   a bond formed like a bridge that bridges the plurality of         abrasive grains one another, and bonding the plurality of         abrasive grains to one another;     -   a plurality of fine grains that are arranged in the bond while         forming a group and that are interposed between the plurality of         abrasive grains so that the plurality of abrasive grains are         arranged so as not to contact one another; and pores formed         around the bond.

In this manner, the plurality of fine grains are arranged forming a group in the bond that bridges and bonds adjacent abrasive grains. Therefore, even when grinding of a workpiece performed by the grinding wheel proceeds, and the bridging portion of the bond is abraded by chips from the workpiece, the plurality of fine grains can fall off one after another. This prevents the plurality of fine grains from being significantly destroyed in a mass at one time. Thus, it is unlikely that the bridging portion of the bond, containing the plurality of fine grains, is destroyed and separated in a mass at one time, impairing the binding force between adjacent abrasive grains and causing the abrasive grains to fall off. Thus, the life of the grinding wheel can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a diagram of a whole grinding wheel, depicting an embodiment of the present invention;

FIG. 2 is a partial enlarged view depicting a structure near a grinding wheel surface of a grinding wheel layer;

FIG. 3 is a detail view depicting a relationship between abrasive grains and a plurality of fine grains depicted in FIG. 2;

FIG. 4 is a flowchart of a method for manufacturing the grinding wheel layer;

FIG. 5 is a partial enlarged view depicting a granulated powder;

FIG. 6 is a partial enlarged view depicting an intermediate molding;

FIG. 7 is a partial enlarged view depicting a pressed molding; and

FIG. 8 is a diagram depicting a state where fine grains are falling off during grinding.

DETAILED DESCRIPTION OF EMBODIMENTS

As depicted in FIG. 1, a grinding wheel 10 is a grinding wheel shaped like a disc. The grinding wheel 10 includes a disc-shaped core 21 and a ring-shaped grinding wheel layer 22. The core 21 is formed of a metal material such as steel, aluminum, or titanium, a fiber reinforced plastic (FRP) material, a ceramic material, or the like. The grinding wheel layer 22 is formed by burning a material into a ring shape and securing the ring-shaped material to an outer periphery of the core 21 with an adhesive or by sintering. Alternatively, the grinding wheel layer 22 may be formed in a ring shape by attaching a plurality of grinding wheel segments to the outer periphery of the core 21.

A center hole 23 is formed in the center of the core 21 so as to pass through the core 21. The center hole 23 is fitted over a centering boss protruding from a shaft end of a wheel spindle of a wheel spindle stock not depicted in the drawings. A plurality of bolt holes 24 (four in the present embodiment) are formed around the center hole 23. Through the bolt holes 24, bolts are inserted which are screwed into screw holes formed in the shaft end of the wheel spindle. The bolts are inserted through the bolt holes 24 and screwed into the screw holes to secure the grinding wheel 10 to the wheel spindle.

As depicted in a partial enlarged view of FIG. 2, the grinding wheel layer 22 includes abrasive grains 12 (in the present embodiment, super abrasive grains 12 of diamond or CBN), a bond 14 (in the present embodiment, a vitrified bond 14), a plurality of fine grains 16 arranged in the bond 14, and pores 18.

As described above, the super abrasive grains 12 (abrasive grains) are, for example, cubic boron nitride (CBN) abrasive grains or diamond abrasive grains. In the present embodiment, an average grain size ΦA of the super abrasive grains 12 is, for example, approximately 125 μm. This size of 125 pm is merely one example, and the average grain size ΦA is not limited to this. As depicted in FIG. 2, the plurality of super abrasive grains 12 are arranged in the grinding wheel layer 22 so as not to contact one another. In this case, an average separation distance L between adjacent super abrasive grains 12 may not be equal to the average grain size ΦA of the super abrasive grains 12, although, in FIG. 2, the average separation distance L is approximately equal to the average grain size ΦA of the super abrasive grains 12. The average separation distance L is set, as appropriate, depending on a desired concentration of the grinding wheel 10.

The vitrified bond 14 (bond), which is well known, bridges and bonds adjacent super abrasive grains 12 to form a bridging portion 20 (see FIGS. 2 and 3). In the bridging portion 20, the above-mentioned fine grains 16 are arranged forming a group and interposed between the plurality of super abrasive grains 12, so that the plurality of super abrasive grains 12 are arranged so as not to contact one another. The plurality of fine grains 16 are made of, for example, alumina (Al₂O₃) that is fine ceramic (ceramic), and are each formed in a substantially spherical shape. However, the fine grains 16 are not limited to this form, and may be formed of another ceramic material. Also, the fine grains 16 may be formed of a material used for a conventional aggregate. The size of each of the fine grains 16 (average grain size ΦB) is equal to or smaller than one-fifth of an average grain size ΦC of a granulated powder 30 that is a group of the plurality of fine grains 16 and will be described later. In the present embodiment, the average grain size 4B of the fine grains 16 is about one-tenth of the average grain size 4C of the granulated powder 30. The shape of each of the fine grains 16 is not limited to a sphere.

In a macroscopic view, the plurality of fine grains 16 in the bridging portion 20 are arranged so that adjacent fine grains 16 contact each other such that a space between adjacent super abrasive grains 12 is filled with the fine grains 16. In a microscopic view, however, there are two states, as depicted in FIG. 3, where adjacent fine grains 16 are separated by a small gap and where adjacent fine grains 16 are in contact with each other. In the case where the small gap is present, the vitrified bond 14 is interposed in the small gap between the adjacent fine grains 16. In the case where the small gap is not present, however, the vitrified bond 14 is not interposed between the adjacent fine grains 16. In any case, adjacent fine grains 16 are bonded to each other by the vitrified bond 14 covering each of the fine grains 16. However, the present invention is not limited to the above aspect. All of adjacent super abrasive grains 12 may have a small gap, or may have no small gap and contact each other, both of which produce the same effect.

As depicted in FIG. 2, the pores 18 are formed around a portion between the super abrasive grains 12 bridged by the vitrified bond 14 (bond). That is, the pores 18 are formed in portions other than the plurality of super abrasive grains 12 and the vitrified bond 14 (containing the plurality of fine grains 16). The pores 18 have a function of temporarily holding chips from a workpiece when the workpiece is ground with the grinding wheel 10.

Now, a method for manufacturing the grinding wheel layer 22 having CBN abrasive grains will be described. The method for manufacturing the grinding wheel layer 22 includes a granulating step S10, a classifying step S12, a mixing step S14, a pressurizing step S16, and a heating step S18, as depicted in the flowchart of FIG. 4.

The granulating step S10 is a step in which, prior to the formation of the grinding wheel layer 22, the plurality of fine grains 16 each formed in a substantially spherical shape are bound to one another for granulation by a binder Bi (binding member), in order to form the plurality of granulated powders 30. Each of the granulated powders 30, as depicted in FIG. 5 that is a partial enlarged view of the granulated powders 30, is formed as a group of the plurality of fine grains 16 bound together. For forming the group, a liquefied binder Bi is caused to adhere to the outer peripheral surface of each of the fine grains 16, and then adjacent portions of the binder Bi are bound to each other. In the present embodiment, each of the granulated powders 30 is substantially spherical. Hereinafter, a diameter of the spherical granulated powder 30 is referred to as “powder diameter”.

Each of the granulated powders 30 is a member that is arranged between adjacent super abrasive grains 12 in the following mixing step S14 and pressurizing step S16, and that determines the average separation distance L between the super abrasive grains 12. In the present embodiment, the average separation distance L between the super abrasive grains 12 is determined based on a desired concentration of the grinding wheel, as described above. Thus, in the granulating step S10, the granulated powders 30 are formed such that the powder diameter of each granulated powder 30 is close to the powder diameter determined based on the concentration of the grinding wheel. The granulated powders 30 having a target average powder diameter ΦC can be extracted in the classifying step S12 that will be described later.

The method for manufacturing the granulated powders 30 may be any method. As one example, a known granulator may be used to manufacture the granulated powders. Specifically, a fluidized bed granulator described in “Zoom Binran (Granulation Handbook)” (compiled by the Association of Powder Process Industry, Japan, 1975.5, published by Ohmsha, Ltd.) may be used to manufacture the granulated powder. The binder Bi required for manufacturing the granulated powders 30 is a binding member that can disappear at a temperature (e.g. below 600° C.) lower than a softening point (e.g. 600° C. or more) of the vitrified bond 14 (bond). In the present embodiment, the binder Bi used is polyvinyl alcohol (PVA), cellulose, or the like. However, any binding member may be used if it satisfies the above temperature condition of disappearance.

In the classifying step S12, a plurality of granulated powders 30 having powder diameters near the average powder diameter ΦC are sorted out from the plurality of granulated powders 30 formed in the granulating step S10. This step is performed so that powder diameters of the granulated powders 30 formed in a spherical shape in the granulating step S10 become a desired and uniform size corresponding to a concentration of the grinding wheel. For example, the plurality of granulated powders 30 are sequentially sifted through different sieves in order of increasing fineness, so that granulated powders 30 having powder diameters near a desired size are extracted (classified). Then, the sorted (classified) granulated powders 30 are supplied to the mixing step S14.

In the mixing step S14, the plurality of granulated powders 30 sorted out in the classifying step S12 and having the average powder diameter 4C are mixed with the plurality of super abrasive grains 12 and powdery vitrified bond 14 x (bond) by using, for example, a known mixer. The above-mentioned super abrasive grains 12 are in a state before the formation of the grinding wheel 10. Thus, the powdery vitrified bond 14 x adheres to the outer peripheral surfaces of the granulated powders 30 and the outer peripheral surfaces of the plurality of super abrasive grains 12, as depicted in FIG. 6. Each of the granulated powders 30 is arranged between adjacent super abrasive grains 12 so that the intermediate molding 32 is formed.

The present invention is not limited to the form described above. The mixing step S14 may have separate steps of causing the powdery vitrified bond 14 x to adhere to the outer peripheral surfaces of the plurality of granulated powders 30, and of causing the powdery vitrified bond 14 x to adhere to the outer peripheral surfaces of the plurality of super abrasive grains 12. In this case, the granulated powders 30 and the super abrasive grains 12 to which the powdery vitrified bond 14 x has adhered may be mixed together by using, for example, a mixer.

In the pressurizing step S16, the intermediate molding 32 (granulated powders 30 +super abrasive grains 13 +vitrified bond 14 x) is fed into a molding die, and the interior of the molding die is then pressurized to form the pressed molding 33 (see FIG. 7). With this, the pressed molding 33 is formed so that adjacent super abrasive grains 12 are arranged separated by a distance of the average powder diameter 4C of the granulated powders 30. That is, adjacent super abrasive grains 12 are arranged separated by the average separation distance L. The pressed molding 33 formed in the pressurizing step S16 is a structure formed by integrally molding the intermediate molding 32 using pressing force. In the present embodiment, the pressed molding 33 is shaped like a ring corresponding to the grinding wheel layer 22.

In the heating step S18, the pressed molding 33 formed in the pressurizing step S16 is heated to produce the grinding wheel layer 22 depicted in FIG. 1. In the heating step S18, the pressed ring-like molding 33 is extracted from a mold frame after the pressurizing step S16, and heated at an appropriate firing temperature (for example, approximately 1,000° C.) of the vitrified bond 14. Subsequently, during the process in which the pressed molding 33 is heated up to the firing temperature, the PVA (binder Bi) that forms the granulated powders 30 by binding disappears at, for example, a temperature of 600° C. or less. As a result, binding member space (not depicted) is created at a position where the PVA has been present, in the interior of each granulated powder 30. At this time, the PVA completely disappears from the granulated powders 30 so that the binding force of the PVA to bind the fine grains 16 to one another is lost. Consequently, each of the granulated powders 30 is changed into a state where the plurality of fine grains 16 faun a group (hereinafter referred to as “group S”). In this case, a small gap may be created between the fine grains 16, as described above. However, since the pressed molding 33 is formed by pressurizing in the pressurizing step S16, the disappearance of the PVA does not significantly deform the group S of the plurality of fine grains 16, which have constituted each of the granulated powders 30.

When the temperature rises to 600° C. or more by heating, the vitrified bond 14 x adhering to the outer peripheral surfaces of the granulated powders 30 is melted, and flows into the binding member space created in the granulated powders 30, and into space between the plurality of fine grains 16 other than the binding member space. Then the melted vitrified bond 14 is cooled and solidified to bond the plurality of fine grains 16, which constituted each of the granulated powders 30 and then constitute the group S.

Similarly, the vitrified bond 14 x adhering to the outer peripheral surfaces of the plurality of super abrasive grains 12 is also melted, and flows toward the group S of the plurality of fine grains 16, which constituted each of the granulated powders 30.

Then the melted vitrified bond 14 flows on a surface of the group S of the plurality of fine grains 16 and bridges adjacent super abrasive grains 12 to form the bridging portion 20. Thus, the group S of the plurality of fine grains 16 is contained in the bridging portion 20. At this time, the group S of the fine grains 16 may or may not be partially exposed from a surface of the bridging portion 20 to the outer space. The pores 18 are formed around a portion between the super abrasive grains 12 bridged by the vitrified bond 14 (bond). In this manner, the ring-like grinding wheel layer 22 is manufactured (see FIG. 2). Subsequently, the sintered grinding wheel layer 22 is secured to the outer periphery of the core 21 using an adhesive to complete the grinding wheel 10.

Next, the effect will be described with reference to FIG. 8. When grinding of a workpiece W is performed with the above-described grinding wheel 10, a chip such as a chip V occurs from the workpiece W. The chip V causes the increase of grinding resistance between the grinding wheel 10 and the workpiece W, in particular, on the bridging portion 20 where adjacent super abrasive grains 12 are bridged by the vitrified bond 14, so that the bridging portion 20 is abraded. Here, the group S of the plurality of fine grains 16 is arranged in the bridging portion 20, in the manufacturing of the grinding wheel 10. The group S has a function of determining the average separation distance L between adjacent super abrasive grains 12. In the group S, adjacent fine grains 16 are bonded to each other by the vitrified bond 14 (bond).

As depicted in FIG. 8, when a surface of the bridging portion 20 on the workpiece W side is abraded, one fine grain 16 of the bridging portion 20 on the workpiece W side, of the plurality of fine grains 16, receives strong resistance from the chip V that has occurred. The fine grain 16 that receives the resistance is one of the plurality of fine grains 16. The one of the fine grains 16 that receives the resistance falls off as the vitrified bond 14 that has been bonding adjacent fine grains 16 to each other is destroyed. Even when another fine grain 16 also receives the strong resistance as the grinding proceeds, only one of the fine grains 16 that receives the resistance falls off due to the same effect. This can eliminate a problematic possibility of the related art in which a single large aggregate is provided between adjacent super abrasive grains 12. Specifically, this can eliminate the possibility that the aggregate receives strong resistance, the aggregate is significantly destroyed and falls off, the vitrified bond having supported the aggregate also falls off along with the aggregate, and then the super abrasive grains fall off within a short time. Thus, the grinding wheel 10 having a long life can be obtained.

According to the above-described embodiment, the grinding wheel 10 includes the plurality of super abrasive grains 12 (abrasive grains) arranged so as not to contact one another, the vitrified bond 14 (bond) formed like a bridge that bridges the plurality of super abrasive grains 12 (abrasive grains) one another, and bonding the plurality of super abrasive grains 12 (abrasive grains) to one another, the plurality of fine grains 16 arranged in the vitrified bond 14 while forming a group and interposed between the plurality of super abrasive grains 12 so that the plurality of super abrasive grains 12 are arranged so as not to contact one another, and the pores 18 formed around the vitrified bond 14.

The plurality of fine grains 16 are arranged forming a group, as an aggregate, in the vitrified bond 14 (bond) that bridges and bonds adjacent super abrasive grains 12 (abrasive grains). Thus, even when grinding of a workpiece performed by the grinding wheel 10 proceeds and the bridging portion 20 of the vitrified bond 14 is abraded by chips from the workpiece, the fine grains 16 can fall off one after another. Since the plurality of fine grains 16 are not significantly destroyed in a mass at one time, it is unlikely that the vitrified bond 14 of the bridging portion 20, containing the plurality of fine grains 16, is separated in a mass at one time, impairing the binding force between the super abrasive grains 12, and causing the super abrasive grains 12 to fall off. Thus, the life of the grinding wheel can be increased.

According to the above-described embodiment, the average grain size ΦB of the plurality of fine grains 16 is one-fifth or less of the average grain size ΦC of the granulated powders 30. Thus, the granulated powders 30 can be suitably formed.

According to the above-described embodiment, the average separation distance L between adjacent two super abrasive grains 12 (abrasive grains), of the plurality of super abrasive grains 12 (abrasive grains), bonded by the vitrified bond 14 is set depending on a concentration of the grinding wheel. Thus, a low-concentration grinding wheel having a significantly reduced concentration can be formed.

According to the above-described embodiment, adjacent fine grains 16 in the group S of the plurality of fine grains 16 are bonded to each other by the vitrified bond 14 (bond). Thus, binding force between adjacent fine grains 16 is sufficiently ensured; and binding force between adjacent super abrasive grains 12 (abrasive grains) bonded through the bridging portion 20 containing the plurality of fine grains 16 is also sufficiently ensured.

According to the above-described embodiment, the plurality of fine grains 16 are made of ceramic having a coefficient of thermal expansion smaller than that of metal. Thus, the plurality of fine grains 16 do not easily thermally expand and contract in the vitrified bond 14 (bond), so that load on the vitrified bond 14 (bond) can be reduced to increase the life of the grinding wheel. Furthermore, ceramic materials with higher versatility can be used, which is economical.

The method for manufacturing the grinding wheel 10 of the above-described embodiment includes: the granulating step S10 that binds, for granulation, the plurality of fine grains 16 to one another by using the binder Bi (binding member) to form the plurality of granulated powders 30; the mixing step S14 that mixes the plurality of granulated powders 30, the plurality of super abrasive grains 12 (abrasive grains) in a state before the formation of the grinding wheel 10, and the vitrified bond 14 (bond) in a powdery state, arranges each of the plurality of granulated powders 30 between adjacent super abrasive grains 12 (abrasive grains) to form the intermediate molding 32; the pressurizing step S16 that feeds the intermediate molding 32 into the molding die and pressurizes the interior of the molding die to form the pressed molding 33; and the heating step S18 that heats the pressed molding 33 such that all the binder Bi (binding member) that forms the granulated powders 30 disappears to form the binding member space between the plurality of fine grains 16, the melted vitrified bond 14 (bond) flows into the binding member space and the space between the plurality of fine grains 16 other than the binding member space, and adjacent super abrasive grains 12 (abrasive grains) are bridged with the vitrified bond 14 containing the plurality of fine grains 16 forming a group that is formed by disappearance of all the binder Bi (binding member) from the granulated powders 30. The above-described grinding wheel 10 can thus be manufactured.

According to the method for manufacturing the grinding wheel 10 of the above-described embodiment, the binding member used in the granulating step S10 is the binder Bi that can disappear at a temperature lower than a softening point of the vitrified bond 14 (bond). Thus, before the heating step S18, the separation distance L between the super abrasive grains 12 (abrasive grains) can be ensured with granulated powders 30. After the start of the heating step S18 in which the firing is performed at a temperature higher than a softening point of the vitrified bond 14 (bond), the binder Bi in the granulated powders 30 disappear, each of the granulated powders 30 is changed into the group S of the plurality of fine grains 16, the melted vitrified bond 14 (bond) flows into the binding member space, from which the binder Bi has disappeared, and into the space other than the binding member space, and then the binding force between the plurality of fine grains 16 and the binding force between adjacent super abrasive grains 12 (abrasive grains) are reliably ensured.

According to the method for manufacturing the grinding wheel 10 of the above-described embodiment, the plurality of granulated powders 30 are formed like a sphere; the plurality of granulated powders 30 formed in the granulating step S10 are sorted out; and the granulated powders 30 having a uniform powder diameter ΦC are extracted. Then, the sorted granulated powders 30 are supplied to the mixing step S14.

In this manner, since powder diameters of the granulated powders 30 that determine the separation distance L between the super abrasive grains 12 (abrasive grains) are uniform, variation of the separation distance L between the super abrasive grains 12 (abrasive grains) is suppressed, and the grinding resistance is maintained stable. The grinding loads on the single grinding wheel are leveled off and the falling-off of the abrasive grains is suppressed so that the tool life can be increased.

According to the above-described embodiment, only the binder Bi is used as the binding member in the granulating step S10 to form the granulated powders 30. However, the present invention is not limited to this form. In the formation of the granulated powders 30, the binding member may be a mixture produced by mixing the liquid binder Bi that can disappear at a temperature lower than a softening point of the vitrified bond 14 x (bond), with the powdery vitrified bond 14 x. Alternatively, the powdery vitrified bond 14 x may be caused to adhere to the surfaces of the fine grains 16 in advance, and then the granulated powders 30 are formed, with the liquid binder Bi dripped, by a granulator. The binder Bi and the vitrified bond 14 x (bond) in such a state may be used as the binding member. In this case, in the heating step S18, first the binder Bi between adjacent fine grains 16, which is part of the binding member, disappears, and then the vitrified bond 14 is melted by further heating to bond the adjacent fine grains 16. That is, the granulated powders 30 are heated, which are formed with the powdery vitrified bond 14 x (bond) incorporated therein in the formation of the granulated powders 30. The vitrified bond 14 x arranged in the vicinity of adjacent fine grains 16 is melted and then solidifies to reliably bond the adjacent fine grains 16. Therefore, stable binding force is obtained between the fine grains 16, and thus stable separation rate of the fine grains 16 is achieved in the grinding.

According to the method for manufacturing the grinding wheel 10 of the above-described embodiment, the classifying step S12 is provided to sort out the granulated powders 30 by the powder diameter. However, the classifying step S12 may not be provided. Even when the granulated powders 30 formed in the granulating step S10 are directly supplied to the subsequent steps, a reasonable effect can also be obtained.

In the above-described embodiment, the grinding wheel 10 is the vitrified bond grinding wheel containing the vitrified bond 14 as a bond. However, the present invention is not limited to this form. In another form, the grinding wheel may be a metal bond grinding wheel formed using a bond containing metal as a main component. In the above-described embodiment, the abrasive grains of the grinding wheel are the super abrasive grains. However, the present invention is not limited to this form. The abrasive grains may be abrasive grains containing alumina or silicon carbide. 

What is claimed is:
 1. A grinding wheel comprising: a plurality of abrasive grains arranged so as not to contact one another; a bond formed like a bridge that bridges the plurality of abrasive grains one another, and bonding the plurality of abrasive grains to one another; a plurality of fine grains that are arranged in the bond while forming a group and that are interposed between the plurality of abrasive grains so that the plurality of abrasive grains are arranged so as not to contact one another; and pores formed around the bond.
 2. The grinding wheel according to claim 1, wherein an average grain size of the plurality of fine grains is equal to or smaller than one-fifth of an average grain size of the group of the plurality of fine grains.
 3. The grinding wheel according to claim 1, wherein an average separation distance between adjacent two abrasive grains, of the plurality of abrasive grains, bonded to each other by the bond is equal to or smaller than an average grain size of the abrasive grains.
 4. The grinding wheel according to claim 1, wherein the plurality of fine grains are bonded to one another by the bond.
 5. The grinding wheel according to claim 1, wherein the plurality of fine grains are made of ceramic.
 6. A method for manufacturing the grinding wheel according to claim 1, comprising: (a) binding, for granulation, the plurality of fine grains to one another with a binding member to form a plurality of granulated powders; (b) mixing the plurality of granulated powders, the plurality of abrasive grains in a state before formation of the grinding wheel, and the bond in a powdery state and arranging each of the plurality of granulated powders between adjacent abrasive grains to form an intermediate molding; (c) feeding the intermediate molding into a molding die and pressurizing the interior of the molding die to form a pressed molding; and (d) heating the pressed molding such that part or all of the binding member forming the granulated powders disappears to form binding member space between the plurality of fine grains, the bond that is melted flows into the binding member space and space between the plurality of fine grains other than the binding member space, and adjacent abrasive grains are bridged with the bond containing the plurality of fine grains forming a group, the group formed by disappearance of part or all of the binding member from the granulated powders.
 7. The method for manufacturing the grinding wheel, according to claim 6, wherein the binding member used in the step (a) is a binder that disappears at a temperature lower than a softening point of the bond.
 8. The method for manufacturing the grinding wheel, according to claim 6, wherein the binding member used in the step (a) is a mixture of a binder that disappears at a temperature lower than a softening point of the bond, and the bond.
 9. The method for manufacturing the grinding wheel, according to claim 6, further comprising: (e) classifying the plurality of granulated powders formed in the step (a) to extract granulated powders having a uniform diameter for supplying the granulated powders to the step (b), wherein the plurality of granulated powders formed in the step (a) are each formed in a generally spherical shape. 